This section introduces aspects that may help facilitate a better understanding of the disclosure. Accordingly, these statements are to be read in this light and are not to be understood as admissions about what is or is not prior art.
Amine- and phosphine-boranes,1,2 the classic Lewis base-Lewis acid (LB-LA) pairs are known for over a century.3 During the initial days, their methods of preparation and properties were the primary focus of chemists.4 Amine-boranes can be readily prepared via ligand exchange using borane-tetrahydrofuran (borane-THF) or borane-methyl sulfide (BMS).1a,5 The low molarity of the former, the stench of the latter and the pyrophoric nature of both are some of the major drawbacks associated with these procedures. In addition, removal of dimethyl sulfide for re-use of the solvent (THF) adds additional steps in the synthetic process with BMS.
Amine-boranes have also been made via transamination.6 The equilibrium in this protocol depends on the nucleophilicity of the incoming amine and can be shifted towards a particular amine-borane by distillation of the volatile amine. One of the problems with this protocol is that the lower boiling amine is only partly removed, thus preventing the completion of the reaction. In certain cases, attempted distillation of the liquid residue has been reported to explode violently.6 Synthesis of amine- and phosphine-boranes from metal hydrides7 and borohydrides8 has also been reported.
Exchange reaction of amine-boranes with phosphines is also known. Hawthorne and coworkers examined the kinetics of the displacement reaction of trialkylamine-boranes with phosphines and proposed an SN2 mechanism at the tetrahedral boron center (SN2B).9 The SN2B reaction kinetics for the amine exchange of ammonia borane (AB, 1) with N,N,N-triethylamine (Et3N) in a flame-sealed NMR tube at 35° C., without a preparative procedure, has been recently described.10 
Depending on the strength of the complexes, a variety of applications have been found for amine-boranes.1 They have been utilized for reduction of carbonyls,11 reductive amination,12 hydrogenation of olefins,13 metal-free hydroboration of alkenes,14 curing agents in epoxy resin preparation,15 preparation of lithium aminoborohydrides,16 or as pharmacologically active moieties,17 etc. Most recently, amine-boranes have gained significance as hydrogen storage materials18 and hypergolic propellants.19 
We had recently reported a near quantitative and safe preparation of a series of aliphatic amine- and phosphine-boranes from AB in refluxing THF via the nucleophilic displacement of ammonia (Scheme 1).20 The volatility of ammonia was exploited during the synthesis.

Large-scale synthesis of ammonia borane became a necessity for the preparation of multi-gram quantities of amine-boranes. The required ammonia borane was prepared in large-scale from sodium borohydride (SBH) and powdered ammonium sulfate in reagent-grade THF containing 5% dissolved ammonia at ambient temperature and pressure (Scheme 2).21

Other procedures to prepare ammonia borane involve the following:
The reaction of SBH and ammonium sulfate under ambient conditions in THF at low concentrations (0.165 M, Scheme 3).22 The high dilution inhibited large-scale preparation. This problem was overcome with an improved synthesis using ammonium formate and SBH in 1 M dioxane (Scheme 4). However, (i) the carcinogenicity of dioxane, (ii) the tedious removal of solvent for recycling, and (iii) the cost of the anhydrous solvent again posed obstacles.

Autrey and coworkers have reported the synthesis of AB from SBH and ammonium chloride in liquid ammonia at −78° C. via THF-mediated decomposition of the in situ generated ammonium borohydride.23 The scale-up of this protocol requires the condensation of large quantities of ammonia. Their reported modification of the metathesis of NaBH4 and NH4Cl in ammonia, followed by the addition of THF is carried out using a Hastelloy reactor. These procedures require anhydrous solvent, low temperature or a special apparatus, thus making the scale-up inconvenient.

More recently, Shore and coworkers reported a large-scale synthesis of AB from either borane-methyl sulfide (BMS) or N,N-dimethylaniline-borane via the nucleophilic displacement of the base with gaseous ammonia in anhydrous THF or toluene.24 The former borane complex (BMS) is expensive, pyrophoric, has an obnoxious odor and is synthesized from SBH using corrosive BF3-etherate. The latter complex (N,N-dimethylaniline-borane) is prepared by passing gaseous diborane, generated in situ from SBH and 12 in diglyme, through N,N-dimethylaniline in toluene (Scheme 5). All of the above mentioned processes are carried out in dry solvents, under an inert atmosphere adding to the cost of AB.

There is therefore an unmet need for novel methods for making ammonia borane and amine-boranes.